Error on misplaced use strict directives

[Copilot]

A "use strict" expression outside a function’s directive prologue is ignored by JavaScript, potentially hiding an incorrectly positioned directive.

Changes

• Report an error when "use strict" appears after executable statements or inside a nested block.
• Apply the check to function declarations, arrows, generators, methods, and constructors.
• Allow preceding type-only declarations because they are erased before JavaScript emit.
• Add compiler coverage for valid and invalid placements.

function example() {
    initialize();
    "use strict"; // Error: directive is not honored here
}

[Ryan Cavanaugh (RyanCavanaugh)]

Just curious what this would do

[Ryan Cavanaugh (RyanCavanaugh)]

TypeScript Bot (@typescript-bot) run top1000

[Ryan Cavanaugh (RyanCavanaugh)]

TypeScript Bot (@typescript-bot) test top1000

[typescript-automation]

Starting jobs; this comment will be updated as builds start and complete.

Command	Status	Results
test top1000	✅ Started	✅ Results

[Copilot]

Pull request overview

Adds compiler diagnostics for misplaced function-level "use strict" directives while permitting erased type declarations.

Changes:

• Detect misplaced directives across function-like bodies.
• Add TS18064 and related compiler coverage.
• Update affected diagnostic and emit baselines.

Show a summary per file

File	Description
tsc/internal/checker/checker.go	Implements placement validation.
tsc/internal/binder/binder.go	Exports strict-directive detection.
tsc/internal/diagnostics/diagnosticMessages.json	Defines TS18064.
tsc/internal/diagnostics/diagnostics_generated.go	Adds generated diagnostic metadata.
tsc/testdata/tests/cases/compiler/useStrictNotInPrologue.ts	Adds placement cases.
tsc/testdata/baselines/reference/compiler/useStrictNotInPrologue.errors.txt	Records expected diagnostics.
tsc/testdata/baselines/reference/compiler/useStrictNotInPrologue.js	Records emit output.
tsc/testdata/baselines/reference/compiler/useStrictNotInPrologue.symbols	Records symbol output.
tsc/testdata/baselines/reference/compiler/useStrictNotInPrologue.types	Records type output.
tsc/testdata/baselines/reference/compiler/strictModeInConstructor.errors.txt	Updates constructor diagnostics.
tsc/testdata/baselines/reference/conformance/functionWithUseStrictAndSimpleParameterList.errors.txt	Updates ES2015 conformance diagnostics.
tsc/testdata/baselines/reference/conformance/functionWithUseStrictAndSimpleParameterList_es2016.errors.txt	Updates ES2016 conformance diagnostics.

Review details

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Files not reviewed (1)

• tsc/internal/diagnostics/diagnostics_generated.go: Generated file

• Files reviewed: 11/12 changed files
• Comments generated: 1
• Review effort level: Balanced

[typescript-automation]

Ryan Cavanaugh (@RyanCavanaugh) Here are the results of running the top 1000 repos with tsc comparing main and refs/pull/64041/merge:

Everything looks good!